Method and apparatus for transmitting reference signal in wireless communication system including relay station

ABSTRACT

A method and an apparatus for demodulating a downlink control channel by a relay node (RN) in a wireless communication system. The method includes receiving cell-specific reference signals on at least one antenna port from a base station (BS); receiving user equipment (UE)-specific reference signals on at least one antenna port from the BS; and demodulating a relay physical downlink control channel (R-PDCCH) based on either the cell-specific reference signals or the UE-specific reference signals. A type of reference signals used to demodulate the R-PDCCH is configured by higher layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of copending application Ser. No.13/384,105, filed on Jan. 13, 2012, which was filed as the NationalStage of PCT International Application No. PCT/KR2010/004696 on Jul. 19,2010, which claims the benefit of priority of U.S. Provisionalapplication No. 61/226,285 filed on Jul. 17, 2009, U.S. Provisionalapplication No. 61/230,118 filed on Jul. 31, 2009, U.S. Provisionalapplication No. 61/254,695 filed on Oct. 25, 2009, and Korean Patentapplication No. 10-2010-0069320 filed on Jul. 19, 2010, all of which areincorporated by reference in their entirety herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications and, moreparticularly, to a method and apparatus for transmitting referencesignals in a wireless communication system including a relay station.

2. Related Art

Effective transmission/reception methods and utilizations have beenproposed for a broadband wireless communication system to maximizeefficiency of radio resources. An orthogonal frequency divisionmultiplexing (OFDM) system capable of reducing inter-symbol interference(ISI) with a low complexity is taken into consideration as one of nextgeneration wireless communication systems. In the OFDM, a serially inputdata symbol is converted into N parallel data symbols, and is thentransmitted by being carried on each of separated N subcarriers. Thesubcarriers maintain orthogonality in a frequency dimension. Eachorthogonal channel experiences mutually independent frequency selectivefading, and an interval of a transmitted symbol is increased, therebyminimizing inter-symbol interference.

When a system uses the OFDM as a modulation scheme, orthogonal frequencydivision multiple access (OFDMA) is a multiple access scheme in whichmultiple access is achieved by independently providing some of availablesubcarriers to a plurality of users. In the OFDMA, frequency resources(i.e., subcarriers) are provided to the respective users, and therespective frequency resources do not overlap with one another ingeneral since they are independently provided to the plurality of users.Consequently, the frequency resources are allocated to the respectiveusers in a mutually exclusive manner. In an OFDMA system, frequencydiversity for multiple users can be obtained by using frequencyselective scheduling, and subcarriers can be allocated variouslyaccording to a permutation rule for the subcarriers. In addition, aspatial multiplexing scheme using multiple antennas can be used toincrease efficiency of a spatial domain.

MIMO technology can be used to improve the efficiency of datatransmission and reception using multiple transmission antennas andmultiple reception antennas. MIMO technology may include a spacefrequency block code (SFBC), a space time block code (STBC), a cyclicdelay diversity (CDD), a frequency switched transmit diversity (FSTD), atime switched transmit diversity (TSTD), a precoding vector switching(PVS), spatial multiplexing (SM) for implementing diversity. An MIMOchannel matrix according to the number of reception antennas and thenumber of transmission antennas can be decomposed into a number ofindependent channels. Each of the independent channels is called a layeror stream. The number of layers is called a rank.

In wireless communication systems, it is necessary to estimate an uplinkchannel or a downlink channel for the purpose of the transmission andreception of data, the acquisition of system synchronization, and thefeedback of channel information. In wireless communication systemenvironments, fading is generated because of multi-path time latency. Aprocess of restoring a transmit signal by compensating for thedistortion of the signal resulting from a sudden change in theenvironment due to such fading is referred to as channel estimation. Itis also necessary to measure the state of a channel for a cell to whicha user equipment belongs or other cells. To estimate a channel ormeasure the state of a channel, a reference signal (RS) which is knownto both a transmitter and a receiver can be used.

A subcarrier used to transmit the reference signal is referred to as areference signal subcarrier, and a subcarrier used to transmit data isreferred to as a data subcarrier. In an OFDM system, a method ofassigning the reference signal includes a method of assigning thereference signal to all the subcarriers and a method of assigning thereference signal between data subcarriers. The method of assigning thereference signal to all the subcarriers is performed using a signalincluding only the reference signal, such as a preamble signal, in orderto obtain the throughput of channel estimation. If this method is used,the performance of channel estimation can be improved as compared withthe method of assigning the reference signal between data subcarriersbecause the density of reference signals is in general high. However,since the amount of transmitted data is small in the method of assigningthe reference signal to all the subcarriers, the method of assigning thereference signal between data subcarriers is used in order to increasethe amount of transmitted data. If the method of assigning the referencesignal between data subcarriers is used, the performance of channelestimation can be deteriorated because the density of reference signalsis low. Accordingly, the reference signals should be properly arrangedin order to minimize such deterioration.

A receiver can estimate a channel by separating information about areference signal from a received signal because it knows the informationabout a reference signal and can accurately estimate data, transmittedby a transmit stage, by compensating for an estimated channel value.Assuming that the reference signal transmitted by the transmitter is p,channel information experienced by the reference signal duringtransmission is h, thermal noise occurring in the receiver is n, and thesignal received by the receiver is y, it can result in y=h·p+n. Here,since the receiver already knows the reference signal p, it can estimatea channel information value ĥ using Equation 1 in the case in which aleast square (LS) method is used.

ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 1]

The accuracy of the channel estimation value ĥ estimated using thereference signal p is determined by the value {circumflex over (n)}. Toaccurately estimate the value h, the value {circumflex over (n)} mustconverge on 0. To this end, the influence of the value {circumflex over(n)} has to be minimized by estimating a channel using a large number ofreference signals. A variety of algorithms for a better channelestimation performance may exist.

Meanwhile, a wireless communication system including a relay station(RS) has recently been developed. A relay station functions to extendthe cell coverage and improve transmission performance. If a basestation (BS) serves UE placed at the boundaries of the coverage of theBS through a relay station, an effect that the cell coverage is extendedcan be obtained. Furthermore, the transmission capacity can be increasedif a relay station improves reliability in signal transmission between aBS and UE. If UE is placed in a shadow region although it is within thecoverage of a BS, the UE may use a relay station. Uplink and downlinkbetween a BS and a relay station is a backhaul link, and uplink anddownlink between a BS and UE or a relay station and UE is an accesslink. Hereinafter, a signal transmitted through the backhaul link iscalled a backhaul signal, and a signal transmitted through the accesslink is called an access signal.

There is a need for a method of efficiently transmitting referencesignals for a relay station.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for transmittingreference signals in a wireless communication system including a relaystation.

In an aspect, a method of transmitting reference signals in a wirelesscommunication system including a relay station is provided. The methodincludes generating a plurality of reference signals for a plurality ofantenna ports, respectively, mapping the plurality of reference signalsto a relay zone within at least one resource block according to apredetermined reference signal pattern, and transmitting the at leastone resource block through the plurality of antenna ports, wherein theplurality of reference signals comprises at least one cell-specificreference signal (CRS) of a 3rd generation partnership project (3GPP)long-term evolution (LTE) Rel-8 system. The relay zone may occupy afourth orthogonal frequency division multiplexing (OFDM) symbol to athirteenth OFDM symbol or a fifth OFDM symbol to a thirteenth OFDMsymbol in one subframe. The plurality of reference signals may be mappedto a relay physical downlink control channel (R-PDCCH) region in which acontrol signal for the relay station is transmitted in the relay zone.The R-PDCCH may occupy first 3 OFDM symbols of the relay zone oroccupies first 3 OFDM symbols of a second slot. A number of the antennaports through which the at least one CRS may be transmitted is one of 1,2, and 4. The plurality of reference signals may further comprise aplurality of relay reference signals for additional antenna ports or ademodulation reference signal (DMRS) of an LTE-advanced (LTE-A) system.The at least one CRS and some of the plurality of relay referencesignals may be mapped by being multiplexed according to a code divisionmultiplexing (CDM) scheme by using an orthogonal code. The method mayfurther include transmitting a reference signal indicator indicatingwhether to demodulate an R-PDCCH by using the at least one CRS or todemodulate an R-PDCCH by using the plurality of relay reference signalsor the DMRS. The reference signal indicator may be transmitted through ahigher layer or subjected to L1/L2 signaling using a PDCCH orbroadcasting.

In another aspect, a method of estimating channels in a wirelesscommunication system including a relay station is provided. The methodincludes receiving a plurality of reference signals through a relay zonewithin a downlink subframe, and performing channel estimation or datademodulation by processing the plurality of reference signals, whereinthe plurality of reference signals comprises at least one cell-specificreference signal (CRS) of a 3rd generation partnership project (3GPP)long-term evolution (LTE) Rel-8 system and comprises any one of aplurality of relay reference signals for additional antenna ports and ademodulation reference signal (DMRS) of an LTE-advanced (LTE-A) system.The plurality of reference signals may be mapped to a relay physicaldownlink control channel (R-PDCCH) region in which a control signal forthe relay station is transmitted in the relay zone. A number of antennaports through which the at least one CRS is transmitted may be one of 1,2, and 4. The performing the channel estimation or the data demodulationmay be based on a reference signal indicator indicating whether todemodulate an R-PDCCH using the at least one CRS or to demodulate anR-PDCCH using the plurality of RN reference signals or the DMRS. Thereference signal indicator may be transmitted through a higher layer orsubjected to L1/L2 signaling using a PDCCH or broadcasting.

In another aspect, an apparatus for estimating channels in a wirelesscommunication system including a relay station is provided. Theapparatus includes a radio frequency (RF) unit configured for receivinga plurality of reference signals through a relay zone within a downlinksubframe, and a processor, coupled to the RF unit, configured forperforming channel estimation or data demodulation by processing theplurality of reference signals, wherein the plurality of referencesignals comprises at least one cell-specific reference signal (CRS) of a3rd generation partnership project (3GPP) long-term evolution (LTE)Rel-8 system and comprises any one of a plurality of relay referencesignals for additional antenna ports and a demodulation reference signal(DMRS) of an LTE-Advanced (LTE-A) system.

A reference signal for a relay station can be defined while reducingsignaling overhead by demodulating a relay-physical downlink controlchannel (R-PDCCH) using the cell-specific reference signal (CRS) of a3rd generation partnership project (3GPP) long-term evolution (LTE)Rel-8 system and the demodulation reference signal (DMRS) of anLTE-advanced (LTE-A) system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows a wireless communication system using a relay station.

FIG. 3 shows the structure of a radio frame in 3GPP LTE.

FIG. 4 shows an example of a resource grid of a single downlink slot.

FIG. 5 shows the structure of a downlink subframe.

FIG. 6 shows the structure of an uplink subframe.

FIGS. 7 to 9 show an exemplary CRS structure.

FIGS. 10 and 11 show examples of a DRS structure.

FIG. 12 is an example of the DMRS structure of an LTE-A system.

FIG. 13 is an example of a downlink subframe to which a relay zone hasbeen allocated.

FIG. 14 is an example of a resource block to which a relay zone has beenallocated.

FIG. 15 is an embodiment of a proposed method of transmitting referencesignals.

FIGS. 16 to 37 are examples of reference signal patterns according toproposed methods of transmitting reference signals.

FIG. 38 is an embodiment of the proposed method of estimating channels.

FIG. 39 is a block diagram showing a BS and a relay station in which theembodiments of the present invention are implemented.

DETAILED DESCRIPTION OF THE INVENTION

As the reference signal sequence, a sequence generated through acomputer based on phase shift keying (PSK) (i.e., a PSK-based computergenerated sequence) may be used. The PSK may include, for example,binary phase shift keying (BPSK), quadrature phase shift keying (QPSK),and the like. Or, as the reference signal sequence, a constant amplitudezero auto-correlation (CAZAC) may be used. The CAZAC sequence mayinclude, for example, a Zadoff-Chu (ZC)-based sequence, a ZC sequencewith cyclic extension, a ZC sequence with truncation, and the like.Also, as the reference signal sequence, a pseudo-random (PN) sequencemay be used. The PN sequence may include, for example, an m-sequence, asequence generated through a computer, a gold sequence, a Kasamisequence, and the like. Also, a cyclically shifted sequence may be usedas the reference signal sequence.

A reference signal can be classified into a cell-specific referencesignal (CRS), an MBSFN reference signal, and a user equipment-specificreference signal (UE-specific RS). The CRS is transmitted to all the UEswithin a cell and used for channel estimation. The MBSFN referencesignal can be transmitted in subframes allocated for MBSFN transmission.The UE-specific reference signal is received by a specific UE or aspecific UE group within a cell, and may be referred to a dedicated RS(DRS). The DRS is chiefly used by a specific UE or a specific UE groupfor the purpose of data demodulation.

First, a CRS is described.

FIGS. 7 to 9 show an exemplary CRS structure. FIG. 7 shows an exemplaryCRS structure when a BS uses one antenna. FIG. 8 shows an exemplary CRSstructure when a BS uses two antennas. FIG. 9 shows an exemplary CRSstructure when a BS uses four antennas. The section 6.10.1 of 3GPP TS36.211 V8.2.0 (2008-03) may be incorporated herein by reference. Inaddition, the exemplary CRS structure may be used to support a featureof an LTE-A system. Examples of the feature of the LTE-A system includecoordinated multi-point (CoMP) transmission and reception, spatialmultiplexing, etc. Furthermore, the CRS maybe used for channel qualityestimation, CP detection and time/frequency synchronization.

Referring to FIG. 7 to FIG. 9, in multi-antenna transmission, a BS usesa plurality of antennas, each of which has one resource grid. ‘R0’denotes an RS for a first antenna, ‘R1’ denotes an RS for a secondantenna, ‘R2’ denotes an RS for a third antenna, and ‘R3’ denotes an RSfor a fourth antenna. R0 to R3 are located in a subframe withoutoverlapping with one another. l indicates a position of an OFDM symbolin a slot. In case of a normal cyclic prefix (CP), l has a value in therange of 0 to 6. In one OFDM symbol, RSs for the respective antennas arelocated with a spacing of 6 subcarriers. In a subframe, the number ofR0s is equal to the number of R1s, and the number of R2s is equal to thenumber of R3s. In the subframe, the number of R2s and R3s is less thanthe number of R0s and R1s. A resource element used for an RS of oneantenna is not used for an RS of another antenna. This is to avoidinterference between antennas.

The CRS is always transmitted by the number of antennas irrespective ofthe number of streams. The CRS has an independent RS for each antenna. Afrequency-domain position and a time-domain position of the CRS in asubframe are determined irrespective of a UE. A CRS sequence to bemultiplied to the CRS is generated also irrespective of the UE.Therefore, all UEs in a cell can receive the CRS. However, a position ofthe CRS in the subframe and the CRS sequence may be determined accordingto a cell identifier (ID). The time-domain position of the CRS in thesubframe may be determined according to an antenna number and the numberof OFDM symbols in a resource block. The frequency-domain position ofthe CRS in the subframe may be determined according to an antennanumber, a cell ID, an OFDM symbol index l, a slot number in a radioframe, etc.

The CRS sequence may be applied on an OFDM symbol basis in one subframe.The CRS sequence may differ according to a cell ID, a slot number in oneradio frame, an OFDM symbol index in a slot, a CP type, etc. The numberof RS subcarriers for each antenna on one OFDM symbol is 2. When asubframe includes N_(RB) resource blocks in a frequency domain, thenumber of RS subcarriers for each antenna on one OFDM symbol is2×N_(RB). Therefore, a length of the CRS sequence is 2×N_(RB).

Equation 2 shows an example of a CRS sequence r(m).

$\begin{matrix}{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Herein, m is 0, 1, . . . , 2N_(RB) ^(max-1). N_(RB) ^(max) denotes thenumber of resource blocks corresponding to a maximum bandwidth. Forexample, when using a 3GPP LTE system, N_(RB) ^(max) is 110. c(i)denotes a PN sequence as a pseudo-random sequence, and can be defined bya gold sequence having a length of 31. Equation 3 shows an example of agold sequence c(n).

c(n)=(x ₁(n+N _(C))+x ₂(n+N _(C)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  [Equation 3]

Herein, NC is 1600, x₁(i) denotes a 1st m-sequence, and x₂(i) denotes a2nd m-sequence. For example, the 1st m-sequence or the 2nd m-sequencecan be initialized for each OFDM symbol according to a cell ID, a slotnumber in one radio frame, an OFDM symbol index in a slot, a CP type,etc.

In case of using a system having a bandwidth narrower than N_(RB)^(max), a certain part with a length of 2×N_(RB) can be selected from anRS sequence generated in a length of 2×N_(RB) ^(max).

The CRS may be used in the LTE-A system to estimate channel stateinformation (CSI). If necessary for estimation of the CSI, channelquality indicator (CQI), a precoding matrix indicator (PMI), a rankindicator (RI), or the like may be reported from the UE.

A DRS is described below.

FIGS. 10 and 11 show examples of a DRS structure. FIG. 10 shows anexample of the DRS structure in the normal CP. In the normal CP, asubframe includes 14 OFDM symbols. R5 indicates the reference signal ofan antenna which transmits a DRS. On one OFDM symbol including areference symbol, a reference signal subcarrier is positioned atintervals of four subcarriers. FIG. 11 shows an example of the DRSstructure in the extended CP. In the extended CP, a subframe includes 12OFDM symbols. On one OFDM symbol, a reference signal subcarrier ispositioned at intervals of three subcarriers. For detailed information,reference can be made to Paragraph 6.10.3 of 3GPP TS 36.211 V8.2.0(2008-03).

The position of a frequency domain and the position of a time domainwithin the subframe of a DRS can be determined by a resource blockassigned for PDSCH transmission. A DRS sequence can be determined by aUE ID, and only a specific UE corresponding to the UE ID can receive aDRS.

A DRS sequence can be obtained using Equations 2 and 3. However, m inEquation 2 is determined by N_(RB) ^(PDSCH). N_(RB) ^(PDSCH) is thenumber of resource blocks corresponding to a bandwidth corresponding toPDSCH transmission. The length of a DRS sequence can be changedaccording to N_(RB) ^(PDSCH). That is, the length of a DRS sequence canbe changed according to the amount of data assigned to a UE. In Equation2, a first m-sequence x₁(i) or a second m-sequence x₂(i) can be resetaccording to a cell ID, the position of a subframe within one radioframe, a UE ID, etc. for every subframe.

A DRS sequence can be generated for every subframe and applied for everyOFDM symbol. It is assumed that the number of reference signalsubcarriers per resource block is 12 and the number of resource blocksis N_(RB) ^(PDSCH), within one subframe. The total number of referencesignal subcarriers is 12×N_(RB) ^(PDSCH). Accordingly, the length of theDRS sequence is 12×N_(RB) ^(PDSCH). In the case in which DRS sequencesaregenerated using Equation 2, m is 0, 1, . . . , 12N_(RB) ^(PDSCH)−1.The DRS sequences are sequentially mapped to reference symbols. The DRSsequence is first mapped to the reference symbol and then to a next OFDMsymbol, in ascending powers of a subcarrier index in one OFDM symbol.

In an LTE-A system, a DRS may be used as a Demodulation Reference Signal(DMRS) for demodulating a PDSCH. That is, the DMRS may be said to be aconcept that the DRS of an LTE Rel-8 system used for beamforming hasbeen expanded into a plurality of layers. The PDSCH and the DMRS maycomply with the same precoding operation. The DMRS may be transmittedonly in a resource block or layer which is scheduled by a BS, andorthogonality is maintained between layers.

FIG. 12 is an example of the DMRS structure of an LTE-A system. The DMRSstructure is the DMRS structure of an LTE-A system supporting fourtransmit antennas in a normal CP structure. A CSI-RS may use the CRS ofan LTE Rel-8 system without change. The DMRS is transmitted in the last2 OFDM symbols (i.e., sixth and seventh OFDM symbols and thirteenth andfourteenth OFDM symbols) of each slot. The DMRS is mapped to first,second, sixth, seventh, eleventh, and twelfth subcarriers within theOFDM symbols in which the DMRS is transmitted.

Further, a CRS can be used together with a DRS. For example, it isassumed that control information is transmitted through three OFDMsymbols (l=0, 1, 2) of a first slot within a subframe. A CRS can be usedin an OFDM symbol having an index of 0, 1, or 2 (l=0, 1, or 2), and aDRS can be used in the remaining OFDM symbol other than the three OFDMsymbols. Here, by transmitting a predefined sequence which is multipliedby a downlink reference signal for each cell, interference betweenreference signals received by a receiver from neighbor cells can bereduced, and so the performance of channel estimation can be improved.The predefined sequence can be one of a PN sequence, an m-sequence, aWalsh hadamard sequence, a ZC sequence, a GCL sequence, and a CAZACsequence. The predefined sequence can be applied to each OFDM symbolwithin one subframe, and another sequence can be applied depending on acell ID, a subframe number, the position of an OFDM symbol, and a UE ID.

In a wireless communication system including a relay station, a relayzone may be defined. The relay zone refers to a section where a controlchannel for a relay station (hereinafter referred to as an R-PDCCH) or adata channel for a relay station (hereinafter referred to as an R-PDSCH)is transmitted within a downlink subframe transmitted by a BS. That is,the relay zone is a section where backhaul transmission is performedwithin a downlink subframe.

FIG. 13 is an example of a downlink subframe to which a relay zone hasbeen allocated.

FIG. 13( a) shows an example of a downlink backhaul subframe transmittedfrom a BS to a relay station or UE. First 3 OFDM symbols are regionsoccupied by PDCCHs transmitted by the BS. In the downlink backhaulsubframe, the relay zone is started from a fourth OFDM symbol, and therelay zone through which an R-PDCCH or an R-PDSCH is transmitted may bemultiplexed with a PDSCH for macro UEs in accordance with a frequencydivision multiplexing (FDM) scheme or a method of combining the FDMscheme and a time division multiplexing (TDM) scheme. A point at whichthe relay zone is started may be determined by the size of an RN PDCCHtransmitted by the relay station. FIG. 13( b) shows an example of adownlink access subframe transmitted from a relay station to UE. When anRN PDCCH transmitted from the relay station to the UE occupies first 2OFDM symbols, a BS may assign a point at which a relay zone is startedin a downlink backhaul subframe as a fourth OFDM symbol. The relay zonemay be allocated in a cell-specific way or an RN-specific way.Furthermore, the relay zone may be allocated dynamically orsemi-persistently. Meanwhile, when the relay station transmits the RNPDCCH through the first 2 OFDM symbols and then receives an uplinkcontrol signal or uplink data from the UE, there is a need for atransition time from transmission to reception. Accordingly, the thirdOFDM symbol may be used as a transition gap.

FIG. 14 is an example of a resource block to which a relay zone has beenallocated. In FIGS. 14( a) and 14(b), the relay zone is allocated from afourth OFDM symbol to a thirteenth OFDM symbol. In FIG. 14( a), thefourth OFDM symbol to the sixth OFDM symbol are a region where anR-PDCCH is transmitted, and the seventh OFDM symbol to the thirteenthOFDM symbol are a region where an R-PDSCH is transmitted. In FIG. 14(b), an eighth OFDM symbol to a tenth OFDM symbol (i.e., the first 3 OFDMsymbols of the second slot of one subframe) are a region where anR-PDCCH are transmitted, and a fourth OFDM symbol to a seventh OFDMsymbol and an eleventh OFDM symbol to a thirteenth OFDM symbol areregions where an R-PDSCH or a PDSCH for a macro LTE-A UE is transmitted.Referring to FIG. 14, the R-PDCCH may be allocated right after the PDCCHregion transmitted by the BS or may be allocated based on the secondslot of the subframe. Meanwhile, in FIG. 14, R0 to R3 refer to resourceelements to which reference signals for the antenna ports 0 to 3 of anLTE Rel-8 system are allocated.

Proposed methods of transmitting reference signals are described belowin connection with embodiments.

A relay station may be introduced into an LTE-A system, and an LTE-Asystem may support a maximum of 8 transmit antennas. The relay stationmay receive a reference signal transmitted by a BS and perform channelestimation or data demodulation. Here, the relay station may use the CRSof an LTE Rel-8 system, the CSI-RS or demodulation reference signal(DMRS) of an LTE-A system, or a new CRS (it may be a DRS used in anLTE-A system on the basis of an LTE Rel-8 system) as the referencesignal. Meanwhile, since a relay zone is allocated within a subframe inorder to transmit an R-PDCCH or an R-PDSCH, a reference signal for arelay station needs to be allocated within the relay zone. Accordingly,a new reference signal pattern different from the existing referencesignal pattern is required.

FIG. 15 is an embodiment of a proposed method of transmitting referencesignals.

At step S100, a BS generates a plurality of reference signals for aplurality of antenna ports. At step S110, the BS maps the plurality ofreference signals to at least one resource block according to apredetermined reference signal pattern. At step S120, the BS transmitsthe at least one resource block to a relay station. When the pluralityof reference signals is mapped to the at least one resource block, theCRS of an LTE Rel-8 system may be used, and reference signals foradditional antenna ports may be additionally mapped to the resourceelement.

The BS may inform the relay station whether the relay station has todecode an R-PDCCH or an R-PDSCH by using the CRS of an LTE Rel-8 systemor the demodulation reference signal (DMRS) of an LTE-A system in orderto demodulate the R-PDCCH or the R-PDSCH. Here, relevant information maybe transmitted through a higher layer or may be subject to L1/L2signaling using a PDCCH or broadcasting. Alternatively, the BS mayinform the relay station whether the relay station has to decode anR-PDCCH or an R-PDSCH by using the CRS of an LTE Rel-8 system or a newlydefined CRS in order to demodulate the R-PDCCH or the R-PDSCH. Here,relevant information may be transmitted through a higher layer or may besubject to L1/L2 signaling using a PDCCH or broadcasting. Types of thereference signals used by the relay station in order to demodulate theR-PDCCH are changed depending on a subframe type or are not changeddynamically. Furthermore, the plurality of reference signals may bemapped to an R-PDCCH region.

Various reference signal patterns to which the proposed method oftransmitting reference signals is applied are described below. In thefollowing description, (a) in the drawings corresponds to a case inwhich an R-PDCCH is allocated to the first N OFDM symbols of a relayzone as in FIG. 14( a), and (b) in the drawings corresponds to a case inwhich the R-PDCCH is allocated to the first N OFDM symbols of the secondslot of a subframe as in FIG. 14( b). Furthermore, in the followingreference signal pattern, a horizontal axis indicates a time domain, anda vertical axis indicates a frequency domain.

First, a case in which the R-PDCCH is started from the fourth OFDMsymbol of a subframe (i.e., a case in which a PDCCH transmitted by a BSoccupies first 3 OFDM symbols) is described.

FIGS. 16 and 17 are examples of reference signal patterns according toproposed methods of transmitting reference signals.

FIGS. 16 and 17 correspond to normal CPs. In this case, CRSs R0 forantenna port 0 may be transmitted through the CRS of an LTE Rel-8system. Resource elements to which the CRS R0 are mapped may comply withFIG. 7. The CRS R0 transmitted in an R-PDCCH region, from among thetransmitted CRS R0, may be used as reference signals for a relaystation. That is, in FIGS. 16( a) and 17(a), the CRS R0 transmitted in afifth OFDM symbol within the R-PDCCH region may be used by the relaystation. In FIGS. 16( b) and 17(b), the CRS R0 transmitted in an eighthOFDM symbol within the R-PDCCH region may be used by the relay station.Reference signals for the remaining antenna ports other than the antennaport 0 may be additionally mapped to resource elements within theR-PDCCH region. The relay station may receive the CRS R0 and theadditionally mapped reference signals and perform channel estimation anddata demodulation.

FIG. 16 is a case in which a maximum of four antennas are supported forthe relay station. N1 to N3 that are reference signals for the relaystation for the antenna ports 1 to 3 other than the CRS R0 may beadditionally mapped to the resource elements within the R-PDCCH region.In FIG. 16( a), N1 may be mapped to the sixth and twelfth subcarriers ofthe fifth OFDM symbol to which the CRS R0 have been mapped. N2 and N3may be mapped to the third subcarrier and the ninth subcarrier,respectively, and the sixth subcarrier and the twelfth subcarrier,respectively, of a fourth OFDM symbol. In FIG. 16( b), N1 may be mappedto the sixth and the twelfth subcarriers of the eighth OFDM symbol towhich the CRS R0 have been mapped. N2 and N3 may be mapped to the thirdsubcarrier and the ninth subcarrier, respectively, and the sixthsubcarrier and the twelfth subcarrier, respectively, of a ninth OFDMsymbol. N1 to N3 to which the reference signals are mapped are notlimited to the reference signal pattern of the present embodiment, butmay be mapped to any resource elements within an R-PDCCH region. Forexample, N2 and N3 may be mapped to a sixth OFDM symbol not the fourthOFDM symbol in FIG. 16( a) and may be mapped to a tenth OFDM symbol notthe ninth OFDM symbol in FIG. 16( b). Furthermore, a subcarrier intervalbetween the reference signals within the OFDM symbol may be adjusted invarious ways and a plurality of reference signals may be transmittedover the entire band within one OFDM symbol.

FIG. 17 is a case in which a maximum of eight antennas are supported forthe relay station. N1 to N7 that are reference signals for the relaystation for the antenna ports 1 to 7 other than the CRS R0 may beadditionally mapped to resource elements within the R-PDCCH region. InFIG. 17( a), N1 may be mapped to the fifth OFDM symbol to which the CRSR0 have been mapped, N2 and N3 may be mapped to a fourth OFDM symbol,and N4 to N7 may be mapped to a sixth OFDM symbol. In FIG. 17( b), N1may be mapped to the eighth OFDM symbol to which the CRS R0 have beenmapped, N2 and N3 may be mapped to a ninth OFDM symbol, and N4 to N7 maybe mapped to a tenth OFDM symbol. The resource elements mapped to N1 toN7 are not limited to the reference signal pattern of the presentembodiment, but may be mapped to any resource elements within an R-PDCCHregion. For example, a subcarrier interval between the reference signalswithin the OFDM symbol may be adjusted in various ways and a pluralityof reference signals may be transmitted over the entire band within oneOFDM symbol.

FIGS. 18 and 19 are other examples of reference signal patternsaccording to proposed methods of transmitting reference signals.

FIGS. 18 and 19 correspond to normal CPs. In this case, CRSs R0 and R1for antenna ports 0 and 1 may be transmitted through the CRS of an LTERel-8 system. Resource elements to which the CRSs R0 and R1 are mappedmay comply with FIG. 8. The CRSs R0 and R1 transmitted in an R-PDCCHregion, from among the transmitted CRSs R0 and R1, may be used asreference signals for a relay station. That is, in FIGS. 18( a) and19(a), the CRSs R0 and R1 transmitted in a fifth OFDM symbol within theR-PDCCH region may be used by the relay station. In FIGS. 18( b) and19(b), the CRSs R0 and R1 transmitted in an eighth OFDM symbol withinthe R-PDCCH region may be used by the relay station. Reference signalsfor the remaining antenna ports other than the antenna ports 0 and 1 maybe additionally mapped to resource elements within the R-PDCCH region.The relay station may receive the CRSs R0 and R1 and the additionallymapped reference signals and perform channel estimation and datademodulation.

FIG. 18 is a case in which a maximum of four antennas are supported forthe relay station. N2 and N3 that are reference signals for the relaystation for antenna ports 2 and 3 other than the CRSs R0 and R1 may beadditionally mapped to resource elements within the R-PDCCH region. InFIG. 18( a), N2 and N3 may be mapped to the third subcarrier and theninth subcarrier, respectively, and the sixth subcarrier and the twelfthsubcarrier, respectively, of a fourth OFDM symbol. In FIG. 18( b), N2and N3 may be mapped to the third subcarrier and the ninth subcarrier,respectively, and the sixth subcarrier and the twelfth subcarrier,respectively, of a ninth OFDM symbol. Resource elements to which N2 andN3 are mapped are not limited to the reference signal patterns of thepresent embodiments, but may be mapped to any resource elements withinan R-PDCCH region. For example, N2 and N3 may be mapped to a sixth OFDMsymbol not the fourth OFDM symbol in FIG. 18( a) and may be mapped to atenth OFDM symbol not the ninth OFDM symbol in FIG. 18( b). Furthermore,a subcarrier interval between the reference signals within the OFDMsymbol may be adjusted in various ways and a plurality of referencesignals may be transmitted over the entire band within one OFDM symbol.

FIG. 19 is a case in which a maximum of eight antennas are supported forthe relay station. N2 to N7 that are reference signals for the relaystation for antenna ports 2 to 7 other than the CRSs R0 and R1 may beadditionally mapped to resource elements within the R-PDCCH region. InFIG. 19( a), N2 and N3 may be mapped to a fourth OFDM symbol, and N4 toN7 may be mapped to a sixth OFDM symbol. In FIG. 19( b), N2 and N3 maybe mapped to the ninth OFDM symbol, and N4 to N7 may be mapped to atenth OFDM symbol. Resource elements to which N2 to N7 are mapped arenot limited to the reference signal pattern of the present embodiment,but may be mapped to any resource elements within an R-PDCCH region. Forexample, a subcarrier interval between the reference signals within theOFDM symbol may be adjusted in various ways and N2 to N7 may betransmitted over the entire band within one OFDM symbol.

FIGS. 20 and 21 are still yet other examples of reference signalpatterns according to proposed methods of transmitting referencesignals.

FIGS. 20 and 21 are normal CPs. In this case, CRSs R0 to R3 for antennaports 0 to 3 may be transmitted through the CRSs of an LTE Rel-8 system.Resource elements to which the CRSs R0 to R1 are mapped may comply withFIG. 9. The CRSs R0 to R3 transmitted in the R-PDCCH region, from amongthe transmitted CRSs R0 to R3, may be used as reference signals for arelay station. That is, in FIGS. 20( a) and 21(a), the CRSs R0 and R1transmitted in a fifth OFDM symbol within an R-PDCCH region may be usedby the relay station. In FIGS. 20( b) and 21(b), the CRSs R0 to R3transmitted in eighth and ninth OFDM symbols within an R-PDCCH regionmay be used by the relay station. Reference signals for the remainingantenna ports for the antenna ports 0 to 3 may be additionally mapped toresource elements within the R-PDCCH region. The relay station mayreceive the CRSs R0 to R3 and the additionally mapped reference signalsand perform channel estimation and data demodulation.

FIG. 20 is a case in which a maximum of four antennas are supported forthe relay station. In FIG. 20( a), N2 and N3 that are reference signalsfor the relay station for the antenna ports 2 and 3 other than the CRSsR0 and R1 may be additionally mapped to resource elements within theR-PDCCH region. N2 and N3 may be mapped to the third subcarrier and theninth subcarrier, respectively, and the sixth subcarrier and the twelfthsubcarrier, respectively, of a fourth OFDM symbol. Resource elements towhich N2 and N3 are mapped are not limited to the reference signalpattern of the present embodiment, but may be mapped to any resourceelements within an R-PDCCH region. For example, N2 and N3 may be mappedto a sixth OFDM symbol not the fourth OFDM symbol in FIG. 20( a) and maybe mapped to a tenth OFDM symbol not the ninth OFDM symbol in FIG. 20(b). Furthermore, for example, a subcarrier interval between thereference signals within the OFDM symbol may be adjusted in various waysand a plurality of reference signals may be transmitted over the entireband within one OFDM symbol. Meanwhile, in FIG. 20( b), since all theCRSs R0 to R3 for the antenna ports 0 to 3 of the LTE Rel-8 system aretransmitted within the R-PDCCH region, reference signals for the relaystation need not to be additionally mapped in order to support a maximumof four antennas.

FIG. 21 is a case in which a maximum of eight antennas are supported forthe relay station. In FIG. 21( a), N2 to N7 that are reference signalsfor the relay station for antenna ports 2 to 7 other than the CRSs R0and R1 may be additionally mapped to resource elements within theR-PDCCH region. N2 and N3 may be mapped to a fourth OFDM symbol, and N4to N7 may be mapped to a sixth OFDM symbol. In FIG. 21( b), N4 to N7that are reference signals for the relay station for antenna ports 4 to7 other than the CRSs R0 to R3 may be additionally mapped to resourceelements within the R-PDCCH region. N4 to N7 may be mapped to a tenthOFDM symbol. The resource elements to which N2 to N7 are mapped are notlimited to the reference signal pattern of the present embodiment, butmay be mapped to any resource elements within an R-PDCCH region. Forexample, for example, a subcarrier interval between the referencesignals within the OFDM symbol may be adjusted in various ways and N2 toN7 or N4 to N7 may be transmitted over the entire band within one OFDMsymbol.

FIGS. 22 to 27 are still yet other examples of reference signal patternsaccording to proposed methods of transmitting reference signals. FIGS.22 to 27 are cases of extended CPs corresponding to FIGS. 16 to 21. Thatis, in FIGS. 22 and 23, CRS R0 for antenna port 0 is transmitted throughthe CRSs of an LTE Rel-8 system. In FIGS. 24 and 25, CRSs R0 and R1 forantenna ports 0 and 1 are transmitted through the CRSs of an LTE Rel-8system. In FIGS. 26 and 27, CRSs R0 to R3 for antenna ports 0 to 3 aretransmitted through the CRSs of an LTE Rel-8 system. Furthermore, amaximum of four antennas are supported for a relay station in FIGS. 22,24, and 26, and a maximum of eight antennas are supported for a relaystation in FIGS. 23, 25, and 27. As in the embodiments described withreference to FIGS. 16 to 21, CRSs transmitted in the R-PDCCH region,from among the CRSs of the LTE Rel-8 system, may be used as referencesignals for the relay station. Reference signals for the remainingantenna ports other than the antenna ports through which the CRSs aretransmitted may be additionally mapped to resource elements within anR-PDCCH region. The relay station may receive the CRSs and theadditionally mapped reference signals and perform channel estimation anddata demodulation. The resource element to which the additionalreference signals are mapped are not limited to the reference signalpatterns of the present embodiments, but may be mapped to any resourceelements within an R-PDCCH region. In the present embodiments,additional reference signals may be mapped to other OFDM symbols withinthe R-PDCCH region other than OFDM symbols to which the additionalreference signals have been mapped. Furthermore, a subcarrier intervalbetween the reference signals within the OFDM symbol may be adjusted invarious ways and a plurality of reference signals may be transmittedover the entire band within one OFDM symbol.

In the above embodiments, the number of OFDM symbols allocated to theR-PDCCH has been assumed to be 3, but the present invention may beapplied to all cases in which the number of OFDM symbols allocated tothe R-PDCCH is 1 or higher. That is, reference signals for a relaystation may be mapped inside the R-PDCCH region or outside the R-PDCCHregion.

Meanwhile, the total number of resource blocks used in downlink in acase in which a bandwidth is 1.4 MHz in an LTE-A system may be 10 orless. Here, A PDCCH transmitted by a BS may be allocated up to the first4 OFDM symbols in a subframe. Accordingly, an R-PDCCH may be startedfrom the fifth OFDM symbol in the subframe. In this case, a referencesignal pattern different from the reference signal patterns of FIGS. 16to 27 is necessary.

FIGS. 28 and 29 still yet other examples of reference signal patternsaccording to proposed methods of transmitting reference signals.

FIGS. 28 and 29 correspond to normal CPs, and they correspond to FIGS.16 and 17. CRS R0 for antenna port 0 is transmitted, and CRS R0transmitted in an R-PDCCH region, from among the transmitted CRSs R0,may be used as reference signals for a relay station. That is, in FIGS.28( a) and 29(a), the CRS R0 transmitted in a fifth OFDM symbol withinthe R-PDCCH region may be used by a relay station. In FIGS. 28( b) and29(b), the CRS R0 transmitted in an eighth OFDM symbol within theR-PDCCH region may be used by the relay station. Reference signals forthe remaining antenna ports other than the antenna ports 0 may beadditionally mapped to resource elements within the R-PDCCH region. Therelay station may receive the CRS R0 and the additionally mappedreference signals and perform channel estimation and data demodulation.FIG. 28 is a case in which a maximum of four antennas are supported forthe relay station, and FIG. 29 is a case in which a maximum of eightantennas are supported for the relay station. Resource elements to whichthe additional reference signals are mapped are not limited to thereference signal patterns of the present embodiments, but may be mappedto any resource elements within an R-PDCCH region. In the presentembodiments, additional reference signals may be mapped to other OFDMsymbols within the R-PDCCH region other than OFDM symbols to which theadditional reference signals have been mapped. Furthermore, a subcarrierinterval between the reference signals within the OFDM symbol may beadjusted in various ways and a plurality of reference signals may betransmitted over the entire band within one OFDM symbol.

FIGS. 30 and 31 are still yet other examples of reference signalpatterns according to proposed methods of transmitting referencesignals.

FIGS. 30 and 31 correspond to normal CPs, and they correspond to FIGS.18 and 19. CRSs R0 and R1 for antenna ports 0 and 1 are transmitted andCRSs R0 and R1 transmitted in an R-PDCCH region, from among thetransmitted CRSs R0 and R1, may be used as reference signals for a relaystation. That is, in FIGS. 30( a) and 31(a), the CRSs R0 and R1transmitted in a fifth OFDM symbol within the R-PDCCH region may be usedby the relay station. In FIGS. 30( b) and 31(b), the CRSs R0 and R1transmitted in an eighth OFDM symbol within the R-PDCCH region may beused by the relay station. Reference signals for the remaining antennaports other than the antenna ports 0 and 1 may be additionally mapped toresource elements within the R-PDCCH region. The relay station mayreceive the CRSs R0 and R1 and the additionally mapped reference signalsand perform channel estimation and data demodulation. FIG. 30 is a casein which a maximum of four antennas are supported for the relay station,and FIG. 31 is a case in which a maximum of eight antennas are supportedfor the relay station. Resource elements to which the additionalreference signals are mapped are not limited to the reference signalpatterns of the present embodiments, but may be mapped to any resourceelements within an R-PDCCH region. In the present embodiments,additional reference signals may be mapped to other OFDM symbols withinthe R-PDCCH region other than OFDM symbols to which the additionalreference signals have been mapped. Furthermore, a subcarrier intervalbetween the reference signals within the OFDM symbol may be adjusted invarious ways and a plurality of reference signals may be transmittedover the entire band within one OFDM symbol.

FIGS. 32 and 33 are still yet other examples of reference signalpatterns according to proposed methods of transmitting referencesignals.

FIGS. 32 and 33 correspond to normal CPs, and they correspond to FIGS.20 and 21. CRSs R0 to R3 for antenna ports 0 to 3 are transmitted, andCRSs R0 to R3 transmitted in an R-PDCCH region, from among thetransmitted CRSs R0 to R3, may be used reference signals for a relaystation. That is, in FIGS. 32( a) and 33(a), the CRSs R0 and R1transmitted in a fifth OFDM symbol within the R-PDCCH region may be usedby the relay station. In FIGS. 32( b) and 33(b), the CRSs R0 to R3transmitted in eighth and ninth OFDM symbols within the R-PDCCH regionmay be used by the relay station. Reference signals for the remainingantenna ports other than the antenna ports 0 to 3 may be additionallymapped to resource elements within the R-PDCCH region. In FIG. 32( b),since the CRSs R0 to R3 for the antenna ports 0 to 3 of an LTE Rel-8system are transmitted within the R-PDCCH region, reference signals fora relay station need not to be additionally mapped in order to support amaximum of four antennas. The relay station may receive the CRSs R0 toR3 and the additionally mapped reference signals and perform channelestimation and data demodulation. FIG. 32 is a case in which a maximumof four antennas are supported for the relay station, and FIG. 33 is acase in which a maximum of eight antennas are supported for the relaystation. Resource elements to which the additional reference signals aremapped are not limited to the reference signal patterns of the presentembodiments, but may be mapped to any resource elements within anR-PDCCH region. In the present embodiments, additional reference signalsmay be mapped to other OFDM symbols within the R-PDCCH region other thanOFDM symbols to which the additional reference signals have been mapped.Furthermore, a subcarrier interval between the reference signals withinthe OFDM symbol may be adjusted in various ways and a plurality ofreference signals may be transmitted over the entire band within oneOFDM symbol.

The embodiments of FIGS. 28 to 33 may also be applied to the case of anextended CP. However, the CRSs transmitted in the first slot cannot beused for a relay station because the first 4 OFDM symbols in thesubframe are allocated to PDCCHs transmitted by a BS. Accordingly, theCRSs R0 and R1 allocated to the fourth OFDM symbol of the first slot maybe used as reference signals for the relay station by replacing the CRSsR0 and R1 with the CRSs R0 and R1 allocated to the first OFDM symbol ofthe second slot. That is, if the PDCCHs occupy the first 4 OFDM symbols,the first OFDM symbol of the second slot may be used as referencesignals for a relay station because the R-PDCCH is allocated from thefifth OFDM symbol. Reference signals for the remaining antenna portsother than antenna ports through which the CRSs are transmitted may beadditionally mapped to resource elements within the R-PDCCH region. Therelay station may receive the CRSs and the additionally mapped referencesignals and perform channel estimation and data demodulation. Resourceelements to which the additional reference signals are mapped are notlimited to the reference signal pattern of the present embodiment, butmay be mapped to any resource elements within an R-PDCCH region. In thepresent embodiment, additional reference signals may be mapped to otherOFDM symbols within the R-PDCCH region other than OFDM symbols to whichthe additional reference signals have been mapped. Furthermore, asubcarrier interval between the reference signals within the OFDM symbolmay be adjusted in various ways and a plurality of reference signals maybe transmitted over the entire band within one OFDM symbol.

Meanwhile, CRSs transmitted by a BS are used as in the embodiments, butthe CRSs and reference signals for additional antenna ports may bemultiplexed using at least one of a code division multiplexing (CDM)scheme, a time division multiplexing (TDM) scheme, and a frequencydivision multiplexing (FDM) scheme. A hybrid multiplexing scheme may beused by combining two or more of the multiplexed schemes.

FIG. 34 is further yet another example of a reference signal patternaccording to a proposed method of transmitting reference signals. InFIG. 34, a plurality of reference signals is multiplexed according theCDM scheme and the plurality of multiplexed reference signals is mappedin the time domain. FIG. 34( a) is a case in which a maximum of fourantennas are supported. Each of CRSs R0 and R2 and CRSs R1 and R3 may bemultiplexed by using a code having a length of 2 as an orthogonal codeaccording to the CDM scheme and then mapped to fourth and fifth OFDMsymbols. Various kinds of codes, such as a DFT code and a Walsh code,may be used as the orthogonal code. Like FIG. 34( a), FIG. 34( b) is acase in which a maximum of eight antennas are supported. Each of (N0,N2), (N1, N3), (N4, N5), and (N6, N7) may be multiplexed by using a codehaving a length of 2 as an orthogonal code according to the CDM schemeand then mapped to fourth and fifth OFDM symbols.

FIG. 35 is further yet another example of a reference signal patternaccording to a proposed method of transmitting reference signals. InFIG. 35, a plurality of reference signals is multiplexed according tothe CDM scheme, and the plurality of multiplexed reference signals ismapped in the frequency domain. FIG. 35( a) is a case in which a maximumof four antennas are supported. Each of N0 and N2 and N1 and N3 may bemultiplexed by using a code having a length of 2 as an orthogonal codeaccording to the CDM scheme and then mapped to a fifth OFDM symbol.Various kinds of codes, such as a DFT code and a Walsh code, may be usedas the orthogonal code. Like FIG. 35( a), FIG. 35( b) is a case in whicha maximum of eight antennas are supported. Each of (N0, N2), (N1, N3),(N4, N5), and (N6, N7) may be multiplexed by using a code having alength of 2 as an orthogonal code according to the CDM scheme and thenmapped to fourth and fifth OFDM symbols, and each of (N0, N2, N4, N5)and (N1, N3, N6, N7) may be multiplexed by using a code having a lengthof 4 as an orthogonal code according to the CDM scheme and then mappedto the fourth and the fifth OFDM symbols.

FIG. 36 is further yet another example of a reference signal patternaccording to a proposed method of transmitting reference signals. InFIG. 36, a plurality of reference signals is multiplexed according tothe FDM scheme. FIG. 36( a) is a case in which a maximum of fourantennas are supported. In FIG. 36( a), CRSs R0 to R3 may be multiplexedaccording to the FDM scheme and mapped to a fifth OFDM symbol. FIG. 36(b) is a case in which a maximum of eight antennas are supported. As inFIG. 36( a), in FIG. 36( a), CRSs R0 to R3 are multiplexed according tothe FDM scheme and mapped to a fifth OFDM symbol, and R4 to R7 may bemultiplexed according to the FDM scheme and mapped to a fourth OFDMsymbol. The position of a subcarrier to which each reference signal ismapped may be changed in various ways.

In the above embodiments, the R-PDCCHs are assumed to occupy 3 OFDMsymbols, but all the embodiments of FIGS. 34 to 36 may be applied to acase in which the R-PDCCHs occupy two or more OFDM symbols.

FIG. 37 is further yet another example of a reference signal patternaccording to a proposed method of transmitting reference signals.Reference signals for a relay station are additionally mapped to a firstOFDM symbol from among OFDM symbols allocated to an R-PDCCH. In FIG. 37,CRSs R0 and R1 may be mapped to a fifth OFDM symbol, and N2 to N7 (i.e.,additional reference signals) may be mapped to a fourth OFDM symbol. InFIG. 37, the R-PDCCH has been assumed to occupy 3 OFDM symbols, but theembodiment of FIG. 37 may be applied to all cases in which the R-PDCCHoccupies 3 or more OFDM symbols.

FIG. 38 is an embodiment of the proposed method of estimating channels.

At step S200, a relay station receives a plurality of reference signalsfrom a BS through the relay zone of a downlink subframe. The pluralityof reference signals may be the CRSs of an LTE Rel-8 system andadditional reference signals for the relay station. Furthermore, theplurality of reference signals may be mapped to an R-PDCCH region. Atstep S210, the relay station performs channel estimation or datademodulation by processing the plurality of reference signals.

FIG. 39 is a block diagram showing a BS and a relay station in which theembodiments of the present invention are implemented.

The BS 800 includes a processor 810, memory 820, and a radio frequency(RF) unit 830. The processor 810 implements the proposed functions,processes and/or methods. The processor 810 generates a plurality ofreference signals for a plurality of antenna ports and maps theplurality of reference signals to at least one resource block accordingto a specific reference signal pattern. When the plurality of referencesignals is mapped to at least one resource block, the CRSs of an LTERel-8 system may be used, and reference signals for additional antennaports may be additionally mapped to resource elements. Furthermore, theplurality of reference signals may be mapped to an R-PDCCH region. Thereference signal patterns of FIGS. 16 to 37 may be formed by theprocessor 810 of the BS 800. The layers of a radio interface protocolmay be implemented by the processor 810. The memory 820 is coupled tothe processor 810, and it stores various pieces of information fordriving the processor 810. The RF unit 830 is coupled to the processor810, and it transmits and/or receives radio signals and transmits the atleast one resource block to a relay station.

The relay station 900 includes a processor 910, memory 920, and an RFunit 930. The RF unit 930 is coupled to the processor 910, and ittransmits and/or receives radio signals and receives a plurality ofreference signals. The plurality of reference signals may be the CRSs ofan LTE Rel-8 system and additional reference signals for the relaystation. Furthermore, the plurality of reference signals may be mappedto an R-PDCCH region. The processor 910 implements the proposedfunctions, processes and/or methods. The processor 910 performs channelestimation or data demodulation by processing the plurality of referencesignals. The layers of a radio interface protocol may be implemented bythe processor 910. The memory 920 is coupled to the processor 910, andit stores various pieces of information for driving the processor 910.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method of demodulating a downlink controlchannel by a relay node (RN) in a wireless communication system, themethod comprising: receiving cell-specific reference signals on at leastone antenna port from a base station (BS); receiving user equipment(UE)-specific reference signals on at least one antenna port from theBS; and demodulating a relay physical downlink control channel (R-PDCCH)based on either the cell-specific reference signals or the UE-specificreference signals, wherein a type of reference signals used todemodulate the R-PDCCH is configured by higher layers.
 2. The method ofclaim 1, further comprising: receiving a reference signal indicator,which indicates the type of reference signals used to demodulate theR-PDCCH, from the BS through the higher-layers.
 3. The method of claim1, wherein a number of the at least one antenna port on which thecell-specific reference signals are received is one of 1, 2 or
 4. 4. Themethod of claim 1, wherein a number of the at least one antenna port onwhich the UE-specific reference signals are received is
 1. 5. The methodof claim 1, wherein the R-PDCCH is mapped to resource elements in atleast one physical resource block (PRB).
 6. The method of claim 1,wherein the R-PDCCH is included in a relay zone used for communicationbetween the RN and the BS.
 7. A relay node (RN) in a wirelesscommunication system, the RN comprising: a radio frequency (RF) unitconfigured to transmit or receive a radio signal; and a processor,coupled to the RF unit, and configured to: receive cell-specificreference signals on at least one antenna port from a base station (BS),receive user equipment (UE)-specific reference signals on at least oneantenna port from the BS, and demodulate a relay physical downlinkcontrol channel (R-PDCCH) based on either the cell-specific referencesignals or the UE-specific reference signals, wherein a type ofreference signals used to demodulate the R-PDCCH is configured by higherlayers.
 8. The RN of claim 7, wherein the processor is furtherconfigured to: receive a reference signal indicator, which indicates thetype of reference signals used to demodulate the R-PDCCH, from the BSthrough the higher-layers.
 9. The RN of claim 7, wherein a number of theat least one antenna port on which the cell-specific reference signalsare received is one of 1, 2 or
 4. 10. The RN of claim 7, wherein anumber of the at least one antenna port on which the UE-specificreference signals are received is
 1. 11. The RN of claim 7, wherein theR-PDCCH is mapped to resource elements in at least one physical resourceblock (PRB).
 12. The RN of claim 7, wherein the R-PDCCH is included in arelay zone used for communication between the RN and the BS.